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03-09-2011, 01:47 PM | #1 | |
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Gas Mileage Myths
We identify and discredit six misconceptions about fuel economy. Quote:
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03-09-2011, 02:55 PM | #2 | |
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While modern sensors (MAF) and computer will adjust the air/fuel mixture on the fly, the engine that has a dirty filter will have to work harder to suck enought of it into the cylinders. This means that there is more load on the engine to achieve the same result as with clean filter. |
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03-09-2011, 02:57 PM | #3 |
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03-09-2011, 02:59 PM | #4 |
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Considering the WRX requires premium (91 or 93) gas, I do wonder what cars are required to use the mid grade 89 stuff, and why it's available everywhere.
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03-09-2011, 03:10 PM | #5 |
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03-09-2011, 03:11 PM | #6 |
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i would disagree with air filter claim
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03-09-2011, 03:17 PM | #7 |
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A fair question: How restricted does an air filter have to be before it starts to reduce an engine's volumetric efficiency enough to produce a measurable reduction in fuel efficiency?
A filter full of dirt will be problematic, but a lightly crudded city filter should go unnoticed. |
03-09-2011, 04:09 PM | #8 |
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03-09-2011, 05:58 PM | #9 |
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Hm, guess I've never come across a car needing anything other than regular or premium.
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03-09-2011, 06:49 PM | #10 | |
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A diesel engine however would gain efficency as there is no air throttle/ |
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03-09-2011, 07:12 PM | #11 | |
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Gas engines have throttles, and whether you restrict the intake air flow at the throttle or at the air filter makes no difference. If you lose 50 mbar across the air filter, you can simply open the throttle a bit further, get the same amount of air into the cylinders and burn the same amount of fuel. edit: aschen was quicker. |
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03-09-2011, 11:07 PM | #12 | |||
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However, as most people have observed after adding a hugeass cone filter to their turbocharged vehicles, the increase in fuel economy and power associated with "high flow" (lower pressure drop) air filter is generally insignificant, as is the reduction with a dirty vs clean filter. There are so many energy losses associated with ICEs and the overall vehicle, that the pumping loss is relatively small fraction of the total energy used. A more efficient configuration is having no throttle, and controlling volumetric efficiency using valve timing only. If you lower the intake pressure drop you can get the same amount of air to flow into the engine in a shorter time, and reduce the time spent "pumping" against a higher pressure differential between the intake and exhaust manifolds. VVVVVVV Quote:
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Last edited by amanichen; 03-09-2011 at 11:43 PM. |
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03-09-2011, 11:27 PM | #13 |
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Even if you throttle with valve timeing you still have "pumping losses" I think for a fixed discplacment engine. I have to think about it more though. The problem is gasoline engines only work in a narrow air fuel range, so at some point if not at max power, you will need to be working against a vacume.
The way to get around this inherent compromise for spark ignition engine is to have one that can operate in lean burn (throttle with amount of fuel injected) or variable discplacement where the intake volume is always atmospheric but you vary the intake stroke or whatever to adjust the power level. Either way, for a typical air throttled engine any restriction upstream of the throttle plate isnt doing any harm until the car is floored. In any part throttle condition it is simply changing the position the user needs to put the pedal to. |
03-09-2011, 11:39 PM | #14 | |
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03-10-2011, 06:40 AM | #15 | |
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If you need an engine to produce say 25 hp to maintain a certain speed, you need to burn a certain amount of air and fuel. If you remove an intake restriction (such as a very dirty air filter) and keep the throttle where it is, the engine will pump more air, inject more fuel and produce more power. That means you now have to close the throttle a bit further to reduce the air flow back to the old level so you can maintain your speed. In other words, the pressure differential will be exactly the same. If what you're saying was true, you could improve your gas mileage by drilling holes into the throttle plate (removing an intake restriction). In reality this will not help fuel economy at all, it will just change the pedal position needed to produce a certain amount of power. |
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03-10-2011, 08:56 AM | #16 | |
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All engines are air pumps, regardless of how much fuel is added. Maintaining a smaller pressure differential across a pump requires less work and less energy. I don't know how else to put it Again, the energy spent pumping air through the engine is a small fraction of the total energy used by an engine. |
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03-10-2011, 09:08 AM | #17 | |
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03-10-2011, 09:22 AM | #18 | ||
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Rallymaniac had it correct in the first reply to this thread. Quote:
Last edited by amanichen; 03-10-2011 at 10:03 AM. |
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03-10-2011, 09:34 AM | #19 | |
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03-10-2011, 09:41 AM | #20 | |
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03-10-2011, 11:00 AM | #21 |
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03-10-2011, 11:10 AM | #22 | |
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No need to go all caps lets keep this classy. I am fully aware the power loss is proportional to the pressure differential multiplied by the volume flow rate....that is the time differential of the PV work (I has a degree in thermo dynamics). However read the replies carfully. Throttleing, or creating a pressure differentional is the means you adjust the power level of a gasoline engine. Half power for instance, requires a certain manifold vacumme lets say -.5 bar. The engine has no idea if this vacume or pressure differential is created at the throttle body or the filter. If you desire -.5psig, the e scenarios below are exactly the same for fuel efficency: 1. No air filter and .5 bar drop at the plate 2. .1bar drop at the filter and .4bar drop at the throttle plat the ONLY difference in the above 2 scenarios is that the pedal is a little closer to the floorboard for #2. The problem with the cloged filter in our obove scenario is if you floor the car and set the throttle plate flow resistance 2 0, you still have drop at the filter. It limits the max opening of the throttle to -.1bar (actually more because the mass flow rate goes up and the pressure drop at the filter will increase similarly). See also, a restrictor plate, doesnt apreciably effect the efficency of a race engine if designed well. It effectively limits how far the throttle can be opened. We get that it takes power to maintain a pressure differential at a flow rate. The point is that you need that vacume anyways to run at part throttle. However you are missing our fundamental argument. Adress this technical instead of resorting to caps........ |
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03-10-2011, 12:13 PM | #23 | |
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If you reduce the overall pressure differential by getting rid of on an intake restriction (such as a dirty air filter), the driver will simply lift the throttle and restore the original pressure differential. That's how part throttle operation works. This it how the picture looks with a dirty air filter: Atmosphere(1bar) --> dirty air filter(-0.1bar) --> partially open throttle (-0.2bar) --> engine (0.7bar abs.) = 35 hp output Now you replce the dirty filter with a clean one: Atmosphere(1bar) --> clean air filter(-0.0bar) --> partially open throttle (-0.2bar) --> engine (0.8bar abs.) = 45 hp output Now the driver would go "wtf, I didn't ask for 45 hp" and lift the throttle because the driving situation only requires 35 hp, and it would look like this: Atmosphere(1bar) --> clean air filter(-0.0bar) --> partially open throttle (-0.3bar) --> engine (0.7bar abs.) = 35 hp output The overall pressure differential is exactly the same as it was with the dirty filter, and so is the air flow, the fuel consumption and the power output. |
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03-10-2011, 12:21 PM | #24 |
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Doesn't it also depend on what fueling mode he's in?
If he's running off the tables, and increase in throttle % (to account for a dirty filter) is going to result in an increase IDC. |
03-10-2011, 12:35 PM | #25 | |
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These engines usually have extremely aggressive cams that would make the MAP signal unreliable, and part throttle driveability/emissions are not important for race engines. |
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